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Abstract Surface meltwater is becoming increasingly widespread on Antarctic ice shelves. It is stored within surface ponds and streams, or within firn pore spaces, which may saturate to form slush. Slush can reduce firn air content, increasing an ice-shelf's vulnerability to break-up. To date, no study has mapped the changing extent of slush across ice shelves. Here, we use Google Earth Engine and Landsat 8 images from six ice shelves to generate training classes using a k -means clustering algorithm, which are used to train a random forest classifier to identify both slush and ponded water. Validation using expert elicitation gives accuracies of 84% and 82% for the ponded water and slush classes, respectively. Errors result from subjectivity in identifying the ponded water/slush boundary, and from inclusion of cloud and shadows. We apply our classifier to the Roi Baudouin Ice Shelf for the entire 2013–20 Landsat 8 record. On average, 64% of all surface meltwater is classified as slush and 36% as ponded water. Total meltwater areal extent is greatest between late January and mid-February. This highlights the importance of mapping slush when studying surface meltwater on ice shelves. Future research will apply the classifier across all Antarctic ice shelves. 

Abstract. In the 2019/2020 austral summer, the surface melt duration andextent on the northern George VI Ice Shelf (GVIIS) was exceptional comparedto the 31 previous summers of distinctly lower melt. This finding is basedon analysis of near-continuous 41-year satellite microwave radiometer andscatterometer data, which are sensitive to meltwater on the ice shelfsurface and in the near-surface snow. Using optical satellite imagery fromLandsat 8 (2013 to 2020) and Sentinel-2 (2017 to 2020), record volumes ofsurface meltwater ponding were also observed on the northern GVIIS in2019/2020, with 23 % of the surface area covered by 0.62 km3 of ponded meltwater on 19 January. These exceptional melt andsurface ponding conditions in 2019/2020 were driven by sustained airtemperatures ≥0 ∘C for anomalously long periods (55 to 90 h)from late November onwards, which limited meltwater refreezing.The sustained warm periods were likely driven by warm, low-speed (≤7.5 m s−1) northwesterly and northeasterly winds and not by foehn windconditions, which were only present for 9 h total in the 2019/2020 meltseason. Increased surface ponding on ice shelves may threaten theirstability through increased potential for hydrofracture initiation; a riskthat may increase due to firn air content depletion in response tonear-surface melting. 

Abstract. Surface meltwater on ice shelves can exist as slush, it can pond in lakes orcrevasses, or it can flow in surface streams and rivers. The collapse of theLarsen B Ice Shelf in 2002 has been attributed to the sudden drainage of∼3000 surface lakes and has highlighted the potential forsurface water to cause ice-shelf instability. Surface meltwater systems havebeen identified across numerous Antarctic ice shelves, although the extentto which these systems impact ice-shelf instability is poorly constrained.To better understand the role of surface meltwater systems on ice shelves,it is important to track their seasonal development, monitoring thefluctuations in surface water volume and the transfer of water acrossice-shelf surfaces. Here, we use Landsat 8 and Sentinel-2 imagery to tracksurface meltwater across the Nivlisen Ice Shelf in the 2016–2017 meltseason. We develop the Fully Automated Supraglacial-Water Tracking algorithmfor Ice Shelves (FASTISh) and use it to identify and track the developmentof 1598 water bodies, which we classify as either circular or linear. Thetotal volume of surface meltwater peaks on 26 January 2017 at 5.5×107 m3. At this time, 63 % of the total volume is held withintwo linear surface meltwater systems, which are up to 27 km long, areorientated along the ice shelf's north–south axis, and follow the surfaceslope. Over the course of the melt season, they appear to migrate away fromthe grounding line, while growing in size and enveloping smaller waterbodies. This suggests there is large-scale lateral water transfer throughthe surface meltwater system and the firn pack towards the ice-shelf frontduring the summer.